Gears are mechanical elements necessary and essential for industrial machines used in industry, agriculture, construction, etc. and for automobiles. These gears are used under tough conditions while subjected to high load, so the bending stress generated at a dedendum is liable to cause the tooth to break.
In the gear cutting process most generally used as the method of production of gears, the dedendum profile, which has a great effect on the dedendum bending strength, is unambiguously determined by the cutting edge profile of the gear cutting tool. Usually, the cutting edge of a gear cutting tool is made as rounded as possible for the purpose of raising the dedendum bending strength, but if excessively rounded, a detrimental effect ends up caused in the meshing of teeth at the time of use of the gears, so there are limits to the improvement of the dedendum bending strength by changing the cutting edge profile.
On the other hand, a gear fabricated by forging or powder metallurgy etc. enables free design of the dedendum profile to an extent not interfering with the addendum of the opposing gear and may enable the fabrication of a gear with a more excellent dedendum bending strength than even a gear fabricated by gear cutting Japanese Patent No. 5520374 (PLT 1) discloses a gear for transmission use produced by hot forging with a tooth root surface comprised of a free curve maximizing the minimum radius of curvature near the dedendum. According to PLT 1, by maximizing the minimum radius of curvature, it is considered that the concentration of stress is eased and the dedendum bending strength is improved.
Further, Japanese Patent Publication No. 2015-1248A (PLT 2) discloses a gear having, from a tooth root to a tooth surface, a first involute part, arc part, second involute part, third involute part, and tooth surface-connecting curve part. According to PLT 2, it is of course considered possible to make the amount of variation of stress in the so-called partially reversed state of tensile stress and compressive stress generated at a tooth root side region uniform and prevent a maximum stress amplitude position from occurring at the tooth root center or its vicinity and also to prevent the maximum stress amplitude position from occurring at the tooth root side region as a whole and possible to improve the durability of a tooth.